Neither Overhead nor Underground, PG&E Pilot Program Evaluates the Benefits of Putting Powerlines Right on the Ground

By Andria Borba

How do you cost-effectively underground overhead electric distribution lines in granite? How do you do it respectfully in culturally sensitive areas? These are some of the questions PG&E coworkers have been asking as the company moves to bury 10,000 miles of powerlines to mitigate wildfire risk.

The answer may lie with a pilot program called Ground-Level Distribution System (GLDS). Instead of trenching three feet down to bury lines, GLDS, as the name implies, places the powerlines on the ground.

But don’t think the lines are exposed. In fact, it’s the opposite as GLDS packages electric cable in conduit in a specially molded tray, tied in with a basalt rebar, then sealed with a special geopolymer cement, placed at ground level and capped in thermoplastic. It sounds complicated, so think of it as a reinforced box-like package that wraps the electric wire in materials to ensure safe operation and is secured to the ground to prevent movement.

First in the world energization

The GLDS pilot project just reached a significant milestone as the first half mile of GLDS circuit was energized in early November on the Woodside 1101 circuit in San Mateo County as the first in-the-world installation and energization.

“Putting 10,000 miles of powerlines underground is an unprecedented effort in the utility industry to reduce wildfire risk,” said Jamie Martin, PG&E’s vice president of Undergrounding. “And we’ve not only made progress in terms of construction and cost efficiency in less than three years, but also we’re focusing on innovation and emerging technologies to complement and enhance our program.”

And that’s where GLDS comes in, said Brad Koelling, senior manager of Grid Design and part of the 10,000-mile Undergrounding program.

“There are places in our system where we need to reduce risk, in which undergrounding is not technically feasible – in areas with hard granite or lava rock. There are also areas where there are cultural and environmental conditions that are not conducive to undergrounding. Before now, the only options were overhead, or dig down three feet,” Koelling said.

The pilot is taking place in Woodside where heavy tree-risk would require significant tree removal to maintain the existing overhead lines in this Tier 3 HFTD area. PG&E and contractor crews installed the GLDS on a private road, needing only easements from property owners, not county permits. In future deployments of GLDS, coordination with state and local agencies will be necessary to obtain approvals.

GLDS can also transition to what is known as Minimum Cable Cover (MCC), which involves only trenching down 10 inches, not the three feet needed with traditional undergrounding. This is part of the appeal of the two programs, according to Koelling. “A lot of the cost and impact of digging is digging, getting that dirt out of there. If it’s contaminated, it costs a lot of money to mitigate. So, how do you get that risk reduction of undergrounding in those environments where it’s not as easy or cost-effective?”

The speed of the process was not only assisted by less trenching, but also through the use of cable in conduit (CIC). Instead of first installing the conduit and then pulling the cables through, the conduit is extruded over the cable, put on a reel and rolled out – eliminating a step in the underground process.

New materials, new ideas

The lynchpin of this project was the use of geopolymer cement. Unlike traditional concrete, this product uses sodium silicate instead of water as a reagent and won’t expand nor contract under a wide range of weather conditions- which means it won’t cause cracks in protective system.

“We don’t normally encase our conduits with a protective medium such as the geopolymer. This geopolymer is a special type of cement that does not transfer heat, that does not use water for the reaction, which provides an extra layer of protection on our primary voltage cables,” said Koelling.

The geopolymer is also fire-resistant and strike-proof — making it especially suited to ground-level work. “Somebody could easily jam a shovel through ordinary conduit, but you’re not going to do that with this geopolymer. The advent and use of the geopolymer really ties it all together,” he explained.

From shovels in the ground to energization, installing GLDS on Woodside 1101 took six weeks.